1. Field of the Invention
The present invention relates to a fixing device and an image forming apparatus incorporating the same, and more particularly, to a fixing device that fixes a toner image in place on a recording medium with heat and pressure, and an electrophotographic image forming apparatus, such as a photocopier, facsimile, printer, plotter, or multifunctional machine incorporating several of those imaging functions, incorporating such a fixing device.
2. Discussion of the Background
In electrophotographic image forming apparatus, such as photocopiers, facsimiles, printers, plotters, or multifunctional machines incorporating several of those imaging functions, an image is formed by attracting toner particles to a photoconductive surface for subsequent transfer to a recording medium such as a sheet of paper. After transfer, the imaging process is followed by a fixing process using a fixing device, which permanently fixes the toner image in place on the recording medium by melting and settling the toner with heat and pressure.
Various types of fixing devices are known in the art, most of which employ a pair of generally cylindrical, looped belts or rollers, one being heated for fusing toner (“fuser member”) and the other being pressed against the heated one (“pressure member”), which together form a heated area of contact called a fixing nip through which a recording medium is passed to fix a toner image under heat and pressure.
One conventional type of fuser assembly employed in the fixing device is an endless belt looped for rotation around a generally cylindrical, stationary heat pipe typically formed by rolling a thin sheet of thermally conductive metal. The heat pipe has a heater inside or outside the pipe hollow to conduct or carry heat over its circumference, from which heat is radially transferred to the length of the fuser belt rotating around the heat pipe. Using a thin-walled conductive heat pipe allows for heating the fuser belt swiftly and uniformly, resulting in shorter periods of warm-up time and first-print time required to complete an initial print job upon startup, and high immunity against printing failures caused by insufficient heating of the fixing nip in high-speed application.
Currently, two different configurations of heat pipes are available for use in electrophotographic fixing devices. One is a generally cylindrical, open-sided roller formed by bending a sheet of thermally conductive material into a rolled configuration with a substantially C-shaped cross-section defining an elongated opening or slit on one side thereof. The other is a completely closed cylindrical roller formed by bending a sheet of thermally conductive material into a rolled configuration, followed by bonding or welding together two opposed longitudinal edges of the rolled sheet to obtain a cylinder with a completely closed cross-section.
A generally cylindrical, open-sided heat pipe is used in combination with a separate fuser pad held stationary in its side opening outside the pipe hollow and inside the loop of a fuser belt entrained around the heat pipe, with adequate spacing left between adjoining surfaces of the heat pipe and the fuser pad. When assembled into a fixing device, the open-sided heat pipe has its open side facing a pressure member extending parallel to the length of the pipe, so that the fuser pad is pressed against the pressure member through the thickness of the fuser belt to form a fixing nip.
On the other hand, a completely closed cylindrical heat pipe is equipped with a reinforcing member held stationary within the pipe hollow against the inner circumference of the pipe for reinforcement purposes. When assembled, the completely closed heat pipe has its outer circumference facing a pressure member extending parallel to the length of the pipe, with the reinforcing member supporting those portions of the pipe circumference pressed against the pressure member to form a fixing nip.
Of the two types of heat pipe described above, the open-sided design is advantageous in terms of protection against deformation under nip pressure. That is, provision of the separate fuser pad enables the open-sided heat pipe to operate substantially in isolation from the pressure member, which can thus maintain its generally cylindrical configuration without bending or bowing away from the fixing nip under nip pressure. Such stability against deformation of the heat pipe in turn protects the fuser belt against damage and failure and results in proper operation of the fixing device, even where the heat pipe is extremely thin-walled to obtain high thermal efficiency in heating the fuser belt.
Although advantaged over its counterpart in terms of mechanical stability, the open-sided heat pipe has a drawback in that it can allow entry of foreign matter into the hollow interior through the side opening, in particular a lubricating agent provided to reduce friction between the adjoining surfaces of the heat pipe and the fuser belt. Leaking lubricant from outside to inside the heat pipe not only results in loss of lubrication, which causes high friction at the interface to aggravate wear and tear of the contacting surfaces, but also results in malfunctioning of or damage to the pipe heater where lubricant adheres to the heater and evaporates in the pipe hollow.
By contrast, the completely closed cylindrical heat pipe is exempt from entry of foreign matter and leakage of lubricant into the pipe hollow, since there is no access to the inside of the pipe from the outside along the circumference of the closed heat pipe.
However, the completely closed heat pipe tends to develop deformation as it is subjected to pressure contact with the pressure member during operation, despite the provision of a reinforcing member supporting the pipe circumference. This tendency toward deformation is pronounced where the heat pipe is formed of an extremely thin sheet of material for obtaining maximum thermal efficiency, where the pressure member applies a higher nip pressure to obtain a longer fixing nip and more efficient fixing performance, and most particularly, where the heat pipe is subjected to varying nip pressure or repeatedly strikes the pressure member as the pressure member moves toward and away from the heat pipe to adjust length and strength (pressure) of the fixing nip.
If not corrected, deformation of the heat pipe results in various defects due to interference or mis-coordination between the fuser belt and the heat pipe, such as the belt getting damaged or making noise by locally and excessively rubbing against the heat pipe. Such defects can be unacceptable where the fixing device incorporates the capability to adjust the length and pressure of the fixing nip by moving the pressure member relative to the heat pipe.
Thus, the two conventional types of heat pipe each has advantages and drawbacks compared to each other in terms of mechanical stability of the cylindrical configuration and immunity against entry of foreign matter into the pipe hollow. As long as this trade-off remains unsolved, neither is satisfactory for providing a reliable high-speed fixing device that can operate with extremely short warm-up time and first-print time, while highly immune to failures caused by insufficient heating of the fuser belt in high speed application.